Indium tin oxide exhibiting high poly-crystallinity on oxygen plasma-treated polyethylene terephthalate surface
© Son et al; licensee Springer. 2012
Received: 11 September 2011
Accepted: 13 February 2012
Published: 13 February 2012
Low-resistivity indium tin oxide [ITO] film was successfully deposited on oxygen plasma-treated polyethylene terephthalate [PET] surfaces at room temperature. X-ray diffraction [XRD] measurements demonstrated that the film deposited on the PET surface that had not been treated with oxygen plasma had an amorphous structure. In contrast, after the low-power oxygen plasma treatment of the PET surface, the ITO film deposited on the PET surface had a poly-crystalline structure due to interactions between electric dipoles on the PET surface and electric dipoles in the ITO film. The minimum resistivity of the poly-crystalline ITO was about 3.6 times lower than that of the amorphous ITO film. In addition, we found that the resistivity of ITO film is proportional to the intensity of the (400) line in the film's XRD spectra.
Transparent conducting oxide [TCO] films have been widely used as transparent electrodes in the field of optoelectronic devices. Specifically, they have been used in applications such as liquid crystal displays, organic light-emitting diodes, e-papers, and thin-film solar cells [1–4]. It is well known that the performance of such devices is closely related to the physical properties (for example, the electrical and optical properties) of the TCO . Indium tin oxide [ITO] is the most widely used TCO because of its excellent electrical and optical characteristics. However, ITO requires high process temperatures (greater than 200°C) in order to obtain these favorable characteristics. This is because an additional annealing treatment is needed to transform its structure from amorphous to poly-crystalline. Thus, it is difficult to apply ITO films onto flexible plastic substrates such as polyethylene terephthalate [PET], polyimide, polyethylene naphthalate, and polycarbonate because these polymer substrates have low glass transition temperatures . In general, there are several disadvantages to fabricating ITO on plastic substrates as opposed to glass substrates, which can be fabricated with a relatively low treatment temperature. These disadvantages include low transmittance, low conductivity, and many defects. Therefore, the development of a low temperature process for depositing ITO onto plastic substrates is very important.
In this work, we present an intriguing technique for fabricating high-performance ITO film on PET without an additional annealing process. Using this method, ITO film with a poly-crystalline structure can be manufactured on oxygen plasma-treated PET surfaces without an additional annealing process. The physical properties of the deposited ITOs on the oxygen plasma-treated PET surfaces are discussed as a function of oxygen plasma power. It is found that the poly-crystalline content of ITO can be optimized by controlling the oxygen plasma power on PET surface.
ITO films were deposited onto non-treated PET using a radio frequency magnetron sputtering system and low-power oxygen plasma-treated PET. Argon (49.7 sccm) was used as the inert gas in the chamber, and oxygen (0.3 sccm) was also used in the chamber. The base pressure of the deposition chamber was around 10-6 Torr, and the working process pressure was around 10-2 Torr. ITO films with a thickness of 50 nm were deposited at room temperature. The oxygen plasma-treated PET surfaces were bombarded by a low-power oxygen gas. The oxygen plasma power, incident angle, and exposure time were 30 to 100 W, 70°, and 5 s, respectively.
X-ray diffraction [XRD] measurements of the SiOx film were performed using an X-ray diffractometer (X'Pert Pro, Philips, PANalytical B.V., Almelo, The Netherlands) equipped with monochromic CuKα radiation (λ = 1.054056 Å) operated at 40 kV and 30 mA. The diffraction pattern was measured at room temperature in normal θ-2θ scanning mode over angles ranging from 10° to 90° with a step of 0.05°, and measurements were performed at a rate of 0.2 s/step.
We also characterized the film's surface morphology using atomic force microscopy [AFM] in the tapping mode (Multimode AFM Nanoscope IIIa, Digital Instruments, Inc., Tonowanda, NY, USA). An ultra-lever cantilever with a spring constant of 26 N/m and a resonance frequency of 268 kHz was used for scanning. Optical transmittance measurements were carried out with an UV-Vis NIR spectrometer.
The wetting properties of the surfaces were determined by the static contact angle method. The contact angles were measured by increasing and then decreasing the volume of a drop of liquid (distilled water) deposited on the sample surface. Recorded images were digitized and analyzed with a software routine that evaluated the tangent at the point of contact between the drop and the surface (i.e., the contact angle).
Measurements to determine the resistivity of the thin films were performed using a four-point probe test. Rectangular sections (3 × 1 cm2) were cut from the substrate. The surfaces of the sections were cleaned thoroughly before the resistivity measurements were made. The four-point probe was placed in contact with the surface of the film, and a fixed current of 10 mA was applied across the outer two probes. The voltage drop across the two inner probes was measured.
Results and discussion
We successfully manufactured ITO that exhibits high poly-crystalline content on oxygen plasma-treated PET surfaces without an additional annealing process. Before the oxygen plasma treatment of the PET surface, the ITO deposited on the PET surfaces exhibited an amorphous structure. In contrast, after the oxygen plasma treatment on PET film, the ITO deposited on the PET surfaces had a poly-crystalline content that depended on the oxygen plasma power. We found that the optimum poly-crystallinity of ITO could be achieved by controlling the oxygen plasma power used on the PET surface. The minimum resistivity of the ITO on the oxygen plasma-treated PET was about 3.6 times lower than that of the ITO on non-treated PET.
This work was supported by a grant from Kyung Hee University in 2011 (KHU-20110184).
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